think of it this way: how many digits are there between 0 and 9 inclusive? 10 right? do we get 10 simply by saying 9 - 0? now, to include the end points when numbering a list, we compute the difference plus 1. so there are 10 digits in a list of integers from 0 to 9 since we have (9 - 0) + 1 = 10

similarly, going from 10 - 99 for instance, we have (99 - 10) + 1 numbers. the 10 here is like the "0" in my previous example. so you are really counting the numbers as "0, 1, 2, 3, ..." as opposed to "1, 2, 3, ..." that +1 accounts for the "0", that is, the first term in the list.